commit 6fd4569a6d4838dba4ea0352e2c76c1eebbf0a9d
Author: tim.schubert@tu-bs.de <tim.schubert@tu-bs.de>
Date:   Wed Oct 26 14:32:50 2016 +0200

    Initial commit.

diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..5b060ef
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,10 @@
+FILE = netcalc
+
+all: $(FILE)
+
+.PHONY: clean
+clean: 
+	rm netcalc
+
+$(FILE): netcalc.c
+	gcc $^ -o $@
diff --git a/netcalc.c b/netcalc.c
new file mode 100644
index 0000000..f706536
--- /dev/null
+++ b/netcalc.c
@@ -0,0 +1,124 @@
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <netdb.h>
+#include <arpa/inet.h>
+#include <netinet/in.h>
+#include <unistd.h>
+
+int cleanup(int sockfd)
+{
+	if (close(sockfd) != 0) {
+		perror("listen");
+		return -1;
+	}
+
+	return 0;
+}
+
+int prepaddr(char *host, char *port, struct addrinfo **ainfo)
+{
+	struct addrinfo hints; // hints to what we want
+	memset(&hints, 0, sizeof hints);
+	hints.ai_family = AF_UNSPEC; // AF_INET or AF_INET6 to force version
+	hints.ai_socktype = SOCK_STREAM;
+	hints.ai_flags = AI_PASSIVE;
+
+	int status;
+	if ((status = getaddrinfo(host, port, &hints, ainfo)) != 0) {
+		fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(status));
+		return -1;
+	}
+	return 0;
+}
+
+void printaddr(struct addrinfo *ainfo)
+{
+
+	char ipstr[INET6_ADDRSTRLEN]; // will hold the IP adress, humanreadable
+	void *addr;
+	char *ipver;
+	if (ainfo->ai_family == AF_INET) { // IPv4
+		struct sockaddr_in *ipv4 = (struct sockaddr_in *) ainfo->ai_addr;
+		addr = &(ipv4->sin_addr);
+		ipver = "IPv4";
+	} else { // IPv6
+		struct sockaddr_in6 *ipv6 = (struct sockaddr_in6 *) ainfo->ai_addr;
+		addr = &(ipv6->sin6_addr);
+		ipver = "IPv6";
+	}
+	inet_ntop(ainfo->ai_family, addr, ipstr, sizeof ipstr);
+	printf("Listening on  %s: %s\n", ipver, ipstr);
+}
+
+int prepsocket(struct addrinfo *ainfo)
+{
+
+	int sockfd = socket(ainfo->ai_family, ainfo->ai_socktype, ainfo->ai_protocol);
+
+	if (sockfd == -1) {
+		perror("socket");
+		return -1;
+	}
+
+	if (bind(sockfd, ainfo->ai_addr, ainfo->ai_addrlen) != 0) {
+		perror("bind");
+		cleanup(sockfd);
+		return -1;
+	}
+
+	int yes = 1;
+	if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof yes) == -1) {
+		perror("setsockopt");
+		cleanup(sockfd);
+		return -1;
+	} 
+
+	if (listen(sockfd, 5) != 0) {
+		perror("listen");
+		cleanup(sockfd);
+		return -1;
+	}
+	return sockfd;
+}
+
+int main(int argc, char *argv[])
+{
+	if (argc != 3) {
+		fprintf(stderr,"usage: netcalc hostname port\n");
+		return 1;
+	}
+
+	struct addrinfo *ainfo;
+	if (prepaddr(argv[1], argv[2], &ainfo) != 0) {
+		fprintf(stderr, "server: failed to obtain adress.");
+		return 1;
+	}
+
+	printaddr(ainfo);
+
+	int sockfd;
+	if ((sockfd = prepsocket(ainfo)) == -1) {
+		return 1;
+	}
+
+	while (1) {
+		struct sockaddr_storage c_addr;
+		socklen_t sin_size = sizeof c_addr;
+		int c_fd = accept(sockfd, (struct sockaddr *)&c_addr, &sin_size);
+		if (c_fd == -1) {
+			perror("accept");
+			continue; // we do not care
+		}
+		if (send(c_fd, "Hello world!\n", 14, 0) == -1) {
+			perror("send");
+		}
+		close(c_fd);
+	}
+
+	cleanup(sockfd);
+	freeaddrinfo(ainfo); // free the linked list
+	return 0;
+}